1. Field of the Invention
This invention relates to a cooling arrangement for the rotor of an electric machine having a superconducting field winding and a co-rotating mixing chamber containing a phase mixture of a coolant supplied from the outside. A coolant distribution system is arranged at the outer circumference of the field winding, and there are cooling canals running through the parts of the field winding between the coolant distribution system and the mixing chamber, as well as coolant connecting lines outside the field winding in which coolant flows by self-pumping through the cooling canals during operation.
2. Description of the Prior Art
A cooling arrangement for the superconducting field winding of a turbo-generator, in which a coolant flow through the field winding takes place under operating conditions due to a self-pumping effect, is described, for instance, in the dissertation of A. Bejan: "Improved Thermal Design of the Cryogenic Cooling System for a Superconducting Synchronous Generator," Ph.D. Thesis, Massachusetts Institute of Technology (USA), December 1974, at pages 148 to 159. The coolant required for cooling the field winding is conducted centrally, in partly liquid condition and at a pressure reduced to below 10.sup.5 Pa, through the rotor shaft by way of a rotating coupler from a refrigeration machine, through a Joule-Thomson valve, and is introduced into a mixing chamber provided there near the axis. A two-phase mixture of liquid and gaseous coolant is then contained in the mixing chamber. Due to the centrifugal forces acting on this two-phase mixture during rotation, the coolant vapor accumulates in the region of the mixing chamber near the axis, and the coolant liquid in the regions away from the axis. A coolant stream with liquid coolant is pumped, from the mixing chamber, via radial canals, into a coolant bath in which the superconducting field winding is arranged, and the heat dissipated by the winding is given off to the coolant. The heat absorbed in the process causes the temperature to rise and the coolant, which is returned to the mixing chamber via further radial canals, to be partly evaporated. In this known machine, a coolant flow through the field winding, which is substantially parallel to the rotor axis, takes place.
The pumping action necessary for developing the flow of the coolant is brought about by a self-pumping effect based on density differences. For, due to centrifugal forces, the coolant is accelerated outward in the radial lines, being isentropically compressed, and is thus pumped into the bath cooling the field winding. In addition, it is warmed up there due by dissipation heat or by heat transferred from the outside, and its density is decreased accordingly. This produces a hydrostatic pressure difference between the radial feed and return lines. Thus, a pressure gradient is developed along the winding between the feed point and the outlet of the coolant. This pressure gradient leads to a convection flow and causes the coolant to be returned via the discharge lines into the mixing chamber near the axis (c.f., "Cryogenics" July 1977, pages 429 to 433, and Germany Offenlegungsschrift 25 30 100). The resulting closed coolant circuit through the field winding is also called a thermo-syphon loop.
In such a thermo-syphon loop, a preferred flow direction does not occur by itself. This leads to difficulties, particularly if a substantially radial flow is to be provided through the field winding of the rotor of a machine. For, particularly during the cooling-down phase of such a field winding, coolant can flow into the cooling canals of the winding directly from the mixing chamber and thus flow through the winding from the inside out. Since the outer parts of the field winding are warmer than the inner parts, but the density of the coolant decreases accordingly, the warmed-up coolant attempts to flow back in to the cooling canals. The danger thus exists that the outer parts of the field winding may not be cooled sufficiently because of instabilities developing within the cooling canals. The field winding can then only be cooled down very slowly and, therefore, uneconomically.
It is therefore an object of the invention to prevent such instabilities in a cooling system which operates by self-pumping in an electric machine having a superconducting field winding through which cooling canals pass, substantially radially, with respect to the rotor axis. In particular, it is an object of the invention to provide a cooling arrangement for a field winding having such cooling canals in which coolant flow from the outside in is always ensured.